1. Field of the Invention
The present invention relates to a semiconductor laser device using a nitride semiconductor, and a method of fabricating such a semiconductor laser device.
2. Description of the Background Art
Based on the usage of nitride semiconductor material of GaN, InN and AlN and a semiconductor of mixed crystals thereof, light emitting devices with InxGa1−N crystal as a light emitting layer have been produced on a sapphire substrate, GaN substrate, SiC substrate, or silicon (111) substrate.
It is to be particularly noted that a silicon substrate is superior than the other substrates by the advantage of providing those of a large area and of constant quality at low cost. Therefore, it is expected that a light emitting device can be fabricated at low cost by using a silicon substrate. Prototypes of semiconductor laser devices are now produced using nitride semiconductor material formed of a semiconductor of such mixed crystals.
By growing a nitride semiconductor using a silicon (111) substrate, a nitride semiconductor film with a C plane as the growth plane can be obtained. However, the planarity of this epitaxial semiconductor film was not so favorable at the atom level.
For example, consider the case where an n type clad layer, a quantum well type light emitting layer formed of InxGa1−x−N, and a p type clad layer are stacked on such a substrate to produce an LD (Laser Diode) of a microstructure. The thickness of the light emitting layer and the In composition are not uniform due to the non-planarity of the film. This unevenness affects the light radiation. It was therefore difficult to obtain induced emission since the emission spectrum has a wide half band width of 40 nm. Only a semiconductor laser device having a high threshold current could be obtained. The obtained device was inferior to a device provided on a sapphire substrate or SiC substrate. It was difficult to obtain a semiconductor laser device of small driving current.
There was another disadvantage of laser using nitride semiconductor material. Specifically, it was found that a semiconductor laser device of low driving current could not be fabricated unless a nitride semiconductor material containing Al that exhibits a large energy gap at respective clad layers formed above and below the active layer is used. The usage of this material allows sufficient light and electrons to be confined in the active layer to emit light at high efficiency.
However, nitride semiconductor material containing Al is disadvantageous in that a crack is easily generated as the crystal grows. It is necessary to prevent generation of cracks in the case where nitride semiconductor material containing Al is used for the clad layers formed above and below the active layer for the purpose of improving the initial characteristics of the threshold value and the like of the semiconductor layer and prevent degradation of the lifetime. The thermal expansion coefficient of silicon is smaller than that of a nitride semiconductor. Therefore, in the case where a silicon substrate is employed, the grown nitride semiconductor is subjected to tensile stress when returned to ambient temperature. There was a problem that a crack is easily generated than when a sapphire substrate or the like is used.